It is increasingly apparent that exercise training brings about significant physiological and biochemical alterations in mammalian systems. Many of these alterations have certain health implications as seen in the prevention of and rehabilitation from coronary heart disease. Cardiovascular adaptations, that significantly reduce cardiac work and provide for a more effective ciculation, are probably subsequent to fundamental changes within the working muscles. This includes a nearly 2-fold increase in the capacity to produce ATP via oxidative metabolism. We have recently found that a change in the synthesis rate is primarily responsible for this adaptation. Studies outlined in this proposal would further clarify the changes in cellular processes which result in the development and maintenance of this increased oxidative capacity. Interest is especially focused on the response of the three different skeletal muscle fiber types, since considerable differences exist in the metabolic and physiologic function of these fibers. One notable effect of endurance training is an increased work capacity. Investigations with muscle stimulated in vitro will be undertaken to explore the physiological significance of the biochemical changes induced by endurance training in the absence of cardiovascular complications. BIBLIOGRAPHIC REFERENCES: Terjung, R.L., and W.W. Winder. Exercise and thyroid function. Med. and Sci. in Sports 7:20-26, 1975. Winder, W.W., K.M. Baldwin, R.L. Terjung and J.O. Holloszy. Effects of thyroid hormone administration on skeletal muscle mitochondria. Am. J. Physiol. 228:1341-1345, 1975.